Facies Distribution and Impact on Petroleum Migration in the Canterbury Basin, New Zealand

Abstract

Canterbury Basin is one of the frontier basins in New Zealand and is under active exploration in multiple permits. Sub-commercial gas and condensate discoveries suggest that it has a working petroleum system. Presence and distribution of petroleum system elements such as source, reservoir and seal rocks within the Cretaceous to Paleocene succession are important components in evaluating the petroleum potential of the basin. This study has used seismic facies analysis to identify these elements. 2D PetroMod modelling has then been used to assess how source rock distribution and carrier bed architecture impacts prospectivity of the offshore Canterbury Basin. Mapping of key sequence stratigraphic surfaces and seismic facies characterisation was carried out to understand basin evolution, facies distribution and depositional environments. Seismic facies were characterised based on seismic amplitude, reflection continuity and geometry of reflection packages and information from five wells. Results were integrated to map facies distribution and to reconstruct source rock distribution and carrier and seal rock architecture for petroleum migration modelling. Facies distribution maps illustrate the mid-Cretaceous to Paleocene evolution of the basin from initial rifting to a subsequent post-rift sag phase. Reservoir facies include fluvial and coastal sandstones in the Cretaceous syn-rift sequences, and mainly shoreface-shelfal sandstones in the Late Cretaceous to Paleocene post-rift section. Late Cretaceous coastal sandstones are restricted to the western margin of the basin whereas shoreface-shelfal sandstones are widely distributed. In accordance with previous studies, Cretaceous coaly facies are modelled to be the primary mature source rocks in the basin. Widely distributed mid-Cretaceous coaly source rocks of the Clipper Formation are mature and peak expulsion is predicted to have occurred during the Paleogene. Late Cretaceous coaly source facies of the Pukeiwitahi Formation are restricted to the southwestern part of the basin and have likely generated petroleum beneath the Plio-Pleistocene shelf margin offlap. Transgressive marine mudstones within the Cretaceous to Eocene sedimentary succession may act as potential seals for underlying reservoirs. Petroleum systems modelling suggests that carrier bed distribution within the Cretaceous and the timing of expulsion relative to seal quality development are key factors controlling basin prospectivity.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015